Diesel engine exhaust is a heterogeneous mixture that contains particulate emissions such as soot and gaseous emissions such as carbon monoxide, unburned or partially burned hydrocarbons, and nitrogen oxides (collectively referred to as NOx). Catalyst compositions, often disposed on one or more monolithic substrates, are placed in engine exhaust systems to convert certain or all of these exhaust components to innocuous compounds.
Ammonia selective catalytic reduction (SCR) is a NOx abatement technology that will be used to meet strict NOx emission targets in diesel and lean-burn engines. In the ammonia SCR process, NOx (consisting of NO+NO2) is reacted with ammonia (or an ammonia precursor such as urea) to form dinitrogen (N2) over a catalyst typically composed of base metals. This technology is capable of NOx conversions greater than 90% over a typical diesel driving cycle, and thus it represents one of the best approaches for achieving aggressive NOx abatement goals.
A characteristic feature of some ammonia SCR catalyst materials is a propensity to retain considerable amounts of ammonia on Lewis and Brønsted acidic sites on the catalyst surface during low temperature portions of a typical driving cycle. A subsequent increase in exhaust temperature can cause ammonia to desorb from the ammonia SCR catalyst surface and exit the exhaust pipe of the vehicle. Overdosing ammonia in order to increase NOx conversion rate is another potential scenario where unreacted ammonia may exit from the ammonia SCR catalyst.
Ammonia slip from the ammonia SCR catalyst presents a number of problems. The odor threshold for NH3 is 20 ppm in air. Eye and throat irritation are noticeable above 100 ppm, skin irritation occurs above 400 ppm, and the IDLH is 500 ppm in air. NH3 is caustic, especially in its aqueous form. Condensation of NH3 and water in cooler regions of the exhaust line downstream of the exhaust catalysts will give a corrosive mixture.
Therefore, it is desirable to eliminate or substantially reduce the ammonia before it can pass into the tailpipe. A selective ammonia oxidation (AMOx) catalyst is employed for this purpose, with the objective to convert the excess ammonia to N2. The ideal catalyst for selective ammonia oxidation will be able to convert ammonia at all temperatures where ammonia slip occurs in the vehicles driving cycle, and will produce minimal nitrogen oxide byproducts. This latter requirement is particularly critical since any production of NO or NO2 by the AMOx catalyst decreases the effective NOx conversion of the exhaust treatment system. The AMOx catalyst should also produce minimal N2O, which is a potent greenhouse gas.
Catalysts comprised of Pt supported on a metal oxide such as γ-alumina are the most active NH3 oxidation catalysts known, exhibiting NH3 lightoff temperatures below 250° C. They are highly effective for the removal of NH3 from a gas stream under oxidizing conditions. However, the selectivity to N2 is not high enough to be applicable in a vehicle emission system. At 250° C., N2 selectivity is less than 50%, with the primary co-product of NH3 oxidation being N2O. As the temperature increases, N2 selectivity decreases. At 450° C., a supported Pt catalyst gives N2 selectivity less than 20%, with the majority of the products consisting of NO and NO2. Hence, there is a desire for ammonia oxidation catalysts with activity comparable to the supported Pt catalysts but with N2 selectivity greater than 60% across the temperature range from 250° C. to 450° C., which is the relevant temperature range for a diesel vehicle driving cycle.